Preparation of silica-alumina catalyst

ABSTRACT

A HIGHLY ACIDIC SILICA-ALUMINA CATALYST, CONTAINING A RELATIVELY LARGE AMOUNT OF ALKALI METAL, E.G. UP TO ABOUT 1%, IS PREPARED BY A PROCESS COMPRISING ACIDIFYING AN ALKALI METAL SILICATE TO A PH OF LESS THAN ABOUT 5.0 TO FORM A SILICA HYDROGEL, WASHING THE HYDROGEL WITH WATER, SLURRYING THE HYDROGEL IN SOLUTION OF AN ALUMINUM SALT, ADDING TO THE SLURRY AN AMMONIACAL BASE IN EXCESS OF THE AMOUNT REQUIRED FOR NEUTRALIZATION OF THE ALUMINUM SALT, AND SEPARATING, DRYING AND CALCINING THE RESULTANT SILICAALUMINA COMPOSITE.

United States Patent US. Cl. 252-451 7 Claims ABSTRACT OF THE DISCLOSURE A highly acidic silica-alumina catalyst, containing a relatively large amount of alkali metal, e.g. up to about 1%, is prepared by a process comprising acidifying an alkali metal silicate to a pH of less than about 5.0 to form a silica hydrogel, washing the hydrogel with water, slurrying the hydrogel in a solution of an aluminum salt, adding to the slurry an ammoniacal base in excess of the amount required for neutralization of the aluminum salt, and separating, drying and calcining the resultant silicaalumina composite.

BACKGROUND OF THE INVENTION Silica-alumina composites are widely used as catalysts and as catalyst carriers for many reactions, including hydrocracking and isomerization. It is well-known that the method of preparing the catalyst may be the determining factor in whether a particular composite may be useful commercially for a given process. Even small variations in the preparation and even small amounts of impurities, particularly the alkali metals, can cause marked changes in a'catalyst with respect to its activity, selectivity and life. Numerous methods have been suggested for preparing such composites. These methods include various cogelling, aging and high temperature steps, and especially the removal of undesirable components, particularly the alkali metal ions. It will be appreciated that each additional step is not only costly in both the time and the labor involved, but also imposes additional technical problems inn the preparation. It is not surprising therefore that the search is still being carried on for more effective catalysts and more economical methods of preparation.

Generally, the source of the activity of the silicaalumina catalysts resides in the acidity produced by the combination of silica and alumina. The presence of alkali metal ions such as the sodium cation normally lowers the total catalyst acidity with concomitant lowering of the catalyst activity. The alkali metal ion which is the main source of trouble is usually sodium since the silica is normally prepared from sodium silicate. Some sodium also may be present in the aluminum salts used in the preparation. Heretofore, silica-alumina formed by known methods have been treated so that the sodium content is reduced to less than 0.10% and generally about 0.02%. Otherwise such catalysts were not acceptable commercially, since they lacked sufiicient activity. In order to reduce the sodium to this low level such treatments as ion exchange steps followed by numerous washing steps are used which, as indicated above, added to the technical and cost problems in the preparation of silica-alumina.

In accordance with this present invention, silicaalumina is prepared by a method which uses no aging or high temperature mixing steps, and moreover, the resultant composite can retain relatively high levels of sodium and still possess high acidity and high activity. The high acidity of the material is evidenced by unusually high 3,65,989 Patented Mar. 21, 1972 p ICC catalytic activity for the cracking of high molecular weight hydrocarbons to lighter compounds even when as much as about 1% sodium is present in the catalyst composition. It is not known exactly why the composite displays this unusual characteristic, but it is believed that in contradistinction to other methods the sodium is in some way locked into the crystal structure and thereby does not affect the acidity of the catalyst.

THE INVENTION [In accordance with another aspect, the present invena highly acidic silica-alumina is prepared by a process comprising acidifying an alkali metal silicate with a mineral acid to a pH of less than about 5.0 to form a silica hydrogel, washing the hydrogel with water, slurrying the hydrogel in a solution of an alumina .salt, adding to the slurry an ammoniacal base in excess of the amount required for neutralization of the aluminum salt, and separating, drying and calcining the resultant silica-alumina deposit.

In accordance with another aspect, the present invention is concerned with the highly active catalyst obtained by the foregoing method, even though it contains relatively large amounts of alkali metal, possibly up to 1%.

The silica component of the silica-alumina composite of this invention is prepared by dispersing an alkali metal silicate in water in an amount corresponding to about 50 to by weight SiO in the resulting dry composite. The alkali metal silicate may be chosen from any one of the known alkali metal silicates. Preferably sodium silicate is used because of its lower cost. The pH of this solution is adjusted to less than about 5 .0. Preferably the pH is adjusted to about 0.5 to 3.5. To adjust the pH any acid may be used provided such acid does not react with the silica. Examples of suitable acids are sulfuric, hydrochloric, nitric, acetic, phosphoric, and trichloracetic. A preferred acid is hydrochloric for the reason of cost.

In contrast with other methods, the silica which precipitates is not permitted to age. It is mixed with the acid at the low pH for a period of about 5 to 30 min., and then filtered. Thereafter the filtrate is reslurried in water, washed and filtered so that the resultant silica hydrogel contains no more than about 2% sodium. However, in clear distincton from conventional catalysts the hydrogel which is used to prepare the final composite may contain up to about 2% sodium.

The composite silica-alumina is formed as follows: The silica hydrogel is dispersed in water and a solution of an aluminum salt is mixed with the silica hydrogel slurry in a sufiicient amount to give a final catalyst containing 10 to 50% A1 0 in the dry composite. The aluminum salt may be, for example, the nitrate, sulfate chloride, or acetate. Preferably the salt is aluminum nitrate because its anion is completely disposable. The aluminum salt solution and silica hydrogel dispersed in water are mixed for a period of 5 to 15 min. Thereafter the composite silica-alumina is formed by adding an ammoniacal base such as ammonium hydroxide, ammonium carbonate, a quaternary ammonium hydroxide, preferably ammoniurn hydroxide, in an amount in excess of that required for complete neutralization of the aluminum salt. Typically an excess of about 10% is used. The resultant composite is filtered.

It has been found that the composite formed in this way can be readily extruded, e.g. through at die at 200-300 p.s.i., or it can be dried to a powder then mulled with water and extruded, or it can be formed into pellets, as by pelletizing or casting.

The catalyst is dried at an elevated temperature, e.g.

300 C. to 800 C., determined by the ultimate use of the composite material.

The resultant composite material may be used as a catalyst, as noted above, for cracking high molecular the oil is complete the system was purged for 15 minutes with nitrogen. The liquid and gaseous products were collected and measured. In such a test the activity of the catalyst is related to the amount of liquid recovered, the

weight hydrocarbons to lighter compounds. However, the lower the amount of liquid recovered, the greater the accomposite is not restricted to such use. It may be used tivity of the catalyst. as a catalyst or catalyst carrier for such reactions of isom- Results of this modified D+L test for typical silicaerization, hydroisomerization, cyclization, and reforming, alumina catalysts prepared according to Example 1 are and indeed for any reaction requiring an acidic catalyst given in the table. carrier material. The following examples will further il- 1.0 AS 6311 be Seen from the table, Silica-alumina Catalysts lustrate the present invention. using a silica gel formed below a pH of 5.0 are consider- EXAMPLE 1 ably more active than those formed above a pH of 5.0. This example illustrates the general method of pre- I EX AMPLE 3 paring silica-alumina composites according to this inven- Thls example further Illustrates the Importance of P tion. at the time of formation of the silica gel.

(a) preparation f the silica gel Following the procedure of Example 1 sodium silicate 1 was acidified with HCl to a pH of 5.8. In this instance, The Slhca gel for Composltmg P ?P Q however, ml. of concentrated HCl was added to the To a Slurry of 2376 grams of sodlllflmlslhcate lters 20 water in the second washing of the gel so that the filtrate f was afjded corlcergrated c at a F of g had a pH of 2.5. After a third water washing and com- 1 to 15 mL/Fnmute m a out 450 of acfld was e positing with alumina to obtain a final catalyst containing and the reaction medium had 1a pH of 2.2. A ter adding 1 10% A12O3 9O% sioz the catalyst was Subjected to thc liter of water and stirrlng for /2 hourlto obtain a uniform D +L test of Example 2 and found to be less active than mlxture the niactlon nmmre fi i T gel 95 catalysts whose silica component was formed below a pH Washed three by reslurrymg each five llte rs H of 5.0. The results of this test are included in the table of water and filtering. The gel so prepared was stored in as catalyst 12 aplastlc bag unmcofnposlted Wlth alumma' b h This test shows that subsequent acidification of the accordance Wlth thls as silica gel is not as significant as the pH at the time of of silica gel were prepared for compositing with alumina, gel formation each batch being prepared at a different pH by varying 3O EXAMPLE 4 the amount of mineral acid added to the sodium silicate.

The high activity of the catalyst of this 1nvent1on com- (b) Preparatlon of slhca'alumma Composlte pared with a commercial cracking catalyst is demonstrated The silica gel was composited with alumina as follows: y thls example To a slurry of 2724 grams of the silica gel in 1250 cc. of A Catalyst Sample of the yp ldehhfied the table water (pH of slurry 2.7) was added a solution of 553 e y l 5 and a eonvenhohal Commefelal eetalyst grams f A1(NO -9H O i 1250 f t Th H identlfied 1n the table as catalyst 15 were subjected to of the aluminum nitrate solution was 1.2 and the slurry, a t0 compare their catalytic eetivhies Cracking after mixture with the aluminum nitrate solution, had a aetlvllly- The test consisted of P g a standard gas Oil H f 15, Aft i i f h 370 f concenover the catalyst at elevated temperature and then flushtrated NI-hOI-I was added The final pH was 7,.5 The reing With nitrogen as in 0111 modified test. In this test hOW- action mixture was stirred for /2 hour, and then without r the P d Was distilled to 3 (g further washing, filtered and extruded through a Ms" die end point, the Coke 011 Catalyst determined, and the at 200300 p.s.i. The extrudate was dried at 110 C. overposition of the gaseous product determined. night and then calcined in air for two hours at 700 C. It was found that despite the fact that the catalysts of to give a composite containing 15% A1 0 and SiO the present invention had a sodium content of about 31 By varying the amount of aluminum nitrate used, comtimes that of the conventional cracking catalyst, activity posites were prepared containing from 10% A1 0 up to was quite comparable, as were the gas compositions and about 50% A1 0 the coke make.

TABLE Composition D+L Test (percent) Calcination Surface pH at Na in area, S102 gel composite, Gas, Liquid, A120, SiO: Hours C. mJ/g. formation percent cc. cc.

10 00 3 600 576 1.4 0.34 106 o.1s- 15 a5 3 600 504 2.2 0.60 226 0.17 10 3 600 530 2.5 0.40 184 0.2 10 00 3 600 543 3.3 0.22 188 0. 22 15 85 2 700 366 2.2 0.63 254 0.1 10 00 3 600 628 2.4 0.17 102 0.2 50 50 4 600 2.3 0.03 270 0.2 50 50 2 500 305 2.3 0.0023 232 0. 2 10 00 3 600 628 4.4 0.17 187 0. 2 10 00 2 600 4.3 0. 22 200 0. 2 10 00 2 600 5.3 0. 48 180 0.3 10 00 3 600 526 5.3 0.95 0.3 10 00 2 600 6.3 0.48 0. 2., 15 85 2 700 533 7.1 1.04 186 0. 4b 13 87 2 600 434 0.02 102 0. 2

B Prepared according to Example 3. b Conventional commercial cracking catalyst.

EXAMPLE 2 The silica-alumina catalysts prepared according to Example l and a standard commercial cracking catalyst were tested for ctivity in a modification of the generally recognized D+L test, which is performed as follows:

One cubic centimeter of a gas oil was charged automatically during a five minute period through a hypodermic needle to a reactor held at 900 F. and containing 5 grams of the silica-alumina catalyst. When addition of (d) adding to the slurry an ammoniacal base in excess of the amount required for neutralization of the aluminum salt, and

(e) separating, drying, and calcining the resultant silica-alumina composite.

2. The method of claim 1 wherein the silica gel is formed at a pH of about 0.5 to 3.5.

3. The method of claim 1 wherein the alkali metal silicate is present in an amount ranging from 50 to 90% by weight based on the SiO in the final silica-alumina composite and the aluminum salt in the slurry is present in the amount ranging from 10 to 50% by weight based on the A1 0 in the final silica-alumina composite.

4. The method of claim 1 wherein the alkali metal silicate is sodium silicate.

5. The method of claim 1 wherein the aluminum salt is aluminum nitrate.

6. The method of claim 1 wherein the ammoniacal basis is ammonium hydroxide.

7. The method of claim 4 wherein the washed silica hydrogel prepared from sodium silicate has a sodium content of up to about 1%.

References Cited UNITED STATES PATENTS DANIEL E. WYMAN, Primary Examiner C. F. DEBS, Assistant Examiner US. Cl. X.R. 252-455 R 

